We are continuing our investigations on the mechanism of estrogen biosynthesis in the human placenta. These studies involve primarily a study of kinetics and isotope effects. We recently have developed a new estrogen synthesizing system from rat ovary by using PMS (pregnant mare serum gonadotropin) stimulation. In investigations to date the properties of the system are similar to the placental system but not identical. We are continuing our studies with the system concentrating on the possible involvement of cytochromes P-450 and b5.